The invention relates to a method of structuring a photoresist layer.
Photolithographic methods for producing integrated circuits on a semiconductor substrate play a key role in semiconductor technology. A radiation-sensitive resist layer is applied to the surface of that layer of substrate that is to be structured. Then, the resist layer is exposed to light of suitable wavelengths in selected parts. Thereafter, only the exposed part of the photoresist layer is removed from the substrate by a suitable developer. The pattern thus produced in the photoresist layer corresponds to the pattern that is to be transferred in a further method step into the substrate layer that is located under the photoresist layer and is to be structured. Examples of further method steps include etching and ion implantation. In this subsequent method step, the developed photoresist layer thus serves as a mask that prevents a change of material, for example ablation of material, in those parts of the substrate layer that are covered by it. After the structuring method step, the photoresist mask is removed and thus does not become part of the integrated circuit.
Resists from the class including the chemical amplification resists (CAR) have proven particularly useful as photoresists. Chemical amplification resists are characterized in that they include a photosensitive acid generator, i.e. a photosensitive compound that generates a protic acid on exposure to light. This protic acid, optionally with thermal treatment of the resist, then initiates acid-catalyzed reactions in the base polymer of the resist. As a result of the presence of the photosensitive acid generator, the sensitivity of the photoresist is substantially increased compared with a conventional photoresist. An overview of this topic is given by H. Ito in Solid State Technology, July 1996, page 164 et seq.
The principle of chemical amplification has become widely used both in the case of one-layer resists developable under wet conditions and in the case of the two-layer resist systems that can be completely or partly developed under dry conditions. In the case of the positive resists, applying the principle of acid-catalyzed cleavage creates the different solubilities of the exposed and of the unexposed photoresists. In acid-catalyzed cleavage, a polar carboxyl group is formed from a nonpolar chemical group of the layer-forming polymer, for example a tert-butyl carboxylate group, in the presence of a photolytically produced acid, optionally in a heating step. Further examples of nonpolar xe2x80x9cblockedxe2x80x9d groups that can be converted into corresponding polar groups by acid-catalyzed reactions are the tert-butoxycarbonyloxy (tBOC) or acetal groups. Through the conversion of the nonpolar group into the corresponding polar group, the resist undergoes a change in polarity in the previously irradiated parts, with the result that it becomes soluble in the polar, aqueous alkaline developer. Consequently, the developer can selectively remove the exposed parts of the photoresist. The resist residues in the unexposed, nonpolar parts thus geometrically define a resist profile or a resist pattern on the substrate, which, in the following method steps, serves as a mask for surface structuring.
Owing to the constantly increasing integration density in semiconductor technology, the accuracy with which the resist profile can be produced after development on a surface to be structured is of decisive importance. On the one hand, the resist profile is physically uniquely predefined by the light distribution during exposure to light. On the other hand, it is chemically transferred to the resist layer by the distribution of the components photochemically produced by the exposure to light.
Owing to the physicochemical properties of the resist materials, completely unfalsified transfer of the pattern predetermined by the lithographic mask to the resist profile is however not uniquely possible. In particular, interference effects and light scattering in the photoresist play a major role here. However, the steps following the exposure, such as the development, also additionally have a great effect on the quality of the resist profiles. The quality of the resist profiles is substantially determined by the profile sidewalls. In order to achieve surface structuring that is as precise as possible in the subsequent method steps, it would be ideal if it were possible to obtain virtually perpendicular, smooth profile sidewalls in the resist profile after the development of the photoresist.
The light intensity profile occurring during the exposure in the photoresist has an adverse effect on the steepness of the profile sidewalls that is to be achieved. This characteristic intensity profile, which is also referred to as xe2x80x9careal imagexe2x80x9d, is due to the light scattering and light absorption occurring in the resist during the exposure to light. Since the photoresist absorbs a certain proportion of the incident radiation, the observed radiation intensity decreases with increasing layer thickness in the photoresist. Consequently, those parts of the photoresist layer that are close to the surface are more strongly exposed to light. In the case of a positive resist, the parts close to the surface are thus more readily soluble than the parts far away from the surface. The different solubilities within an exposed part of the resist often lead, in the case of positive resists, to flattening and to poor definition of the profile sidewalls. The light intensity profile in the photoresist describes the distribution of a photochemically changed species, for example, in the case of a positive resist, the distribution of the photolytically produced acid in the photoresist.
The quality and the steepness of the resist profiles are of decisive importance for transferring the structure from the photomask to the layer that is present underneath and is to be structured. A known approach for improving the quality of resist profiles in positive resists is described in European Patent Application EP 0 962 825 A1. In which, an improved steepness of the resist sidewalls is achieved by adding to the photoresist two photochemically active additives that are activated by radiation in respective different wavelength ranges.
On the other hand, the photoresist contains a photosensitive acid generator that, as already described above, liberates an acid on exposure to light of a defined wavelength range. The liberated acid then catalyzes the reaction of the convertible nonpolar groups of the layer-forming polymer of the photoresist into carboxyl groups and thus causes the photoresist to be soluble in the polar developer.
On the other hand, the photoresist contains, as a second photochemical additive, a crosslinking reagent that results in a reduction in the solubility of the photoresist. This crosslinking reagent is likewise activated by radiation. The radiation used for this purpose differs from the radiation used for activating the photosensitive acid generator.
In a first structuring exposure step in this method, the photosensitive acid generator is activated in the parts determined by the mask layout. In a subsequent, second floodlight exposure step, the total photoresist layer is exposed without the use of a photomask and hence the crosslinking reagent is photochemically activated over the entire area of the photoresist layer. As a result of the consequently initiated chemical crosslinking of the photoresist, its solubility is reduced. Because those parts of the photoresist that are close to the surface are more strongly exposed to light, they are more highly crosslinked and hence more insoluble than the parts far away from the surface. Through this selective solubility modification in the photoresist, higher developer selectivity in the aqueous developer is achieved, with the result that steeper resist profile sidewalls are obtained.
However, this approach has a decisive disadvantage because the crosslinking reaction leads to the formation of a three-dimensional network polymer, in particular in those parts of the photoresist that are close to the surface. This network polymer has changed development behavior compared with the original, linear layer-forming polymer, which leads to xe2x80x9croughxe2x80x9d, i.e. inexactly defined, for example frayed profile sidewalls. This roughness complicates the subsequent method steps, such as the etching of the substrate. Moreover, this method requires the use of two different photochemically active compounds that must be tailored both to one another and to the base polymer.
It is accordingly an object of the invention to provide a method of structuring a photoresist layer that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that structures a photoresist layer that reduces or entirely avoids the disadvantages described above. In particular, it is the object of the present invention to provide a method by which high accuracy of transfer of the structure predetermined by the lithographic mask to a photoresist layer is achieved.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method of structuring a photoresist layer. The first step is providing a substrate. The next step is applying a photoresist layer at least partially to the substrate. The photoresist layer includes a film-forming polymer having molecular groups that is convertible into alkali-soluble groups at a temperature by acid-catalyzed cleavage reactions. The photoresist layer also includes a photosensitive acid generator for liberating an acid when exposed to light in a defined wavelength range. The next step is partially exposing the photoresist layer to light in the defined wavelength range. The next step is contacting the photoresist layer with a base over a specific period. The next step is allowing the base to diffuse into the photoresist layer. The next step is heating the photoresist layer to the temperature to cause the cleavage reaction catalyzed by the photolytically produced acid. The next step is developing the photoresist layer.
According to the invention, a method of structuring a photoresist layer is provided that includes the following steps. A substrate is provided on which a photoresist layer has been applied at least in parts. The photoresist layer includes a film-forming polymer that has molecular groups that can be converted into alkali-soluble groups by acid-catalyzed cleavage reactions. Furthermore, the polymer includes a photosensitive acid generator that liberates an acid on exposure to light from a defined wavelength range. The photoresist layer is exposed in parts to light from the defined wavelength range and is brought into contact, over a specific period, with a base that is soluble in the photoresist layer. It is not important whether the photoresist layer is brought into contact with the base after the irradiation or whether this is effected before the irradiation. The photoresist layer is then heated to a temperature at which the cleavage reaction catalyzed by the photolytically produced acid takes place. The photoresist layer is then developed.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of structuring a photoresist layer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.